This invention relates to X-ray tubes and, more particularly, a two-piece cathode cup for X-ray tubes and a method of manufacturing the cups.
Diagnostic X-ray equipment has been in use for many years. During recent years, new types of X-ray equipment have become available, and the standards of performance relating to both those new types of X-ray equipment as well as classical radiographic systems have been upgraded. Many of the improved performance characteristics rely upon X-ray tubes having small focal spots, which in turn require the electron beam within the tube be well controlled and focused. In addition, certain applications and apparatus require that the X-ray beam be grid controlled so that the beam itself, and hence the emission of X-rays, can be interrupted during equipment operation.
The ability to carefully control the electron beam has been dependent upon positioning the electron-emitting filament in what is known as a cathode cup. The cathode cup has beam-focusing surfaces around the filament.
Traditionally, the X-ray tubes are manufactured by setting filaments deep within a recess in one-piece cathode cups. Following this procedure, certain "tabs" are added to provide additional beam-focusing surfaces.
Problems which have been found with this prior-art construction technique include the difficulty of accurately positioning the filament when the assembler must work within the recess. Misaligned filaments negatively affected the precision of beam focus which could be obtained.
Additionally, the requirement that the operator perform the task within the recess sometimes resulted in scratches or nicks on the otherwise smooth outer surface of the cathode cup. Such damage to the cup could cause sharp points of metal. These are significant because X-ray tubes often utilize voltage differences in excess of 100 kv. between the cathode and the anode. As is well known, electric field lines are concentrated near sharp points and, thus, internal arcing can result from the presence of sharp points on the cathode cup.
Finally, the tabs, which had to be set down within the recess in the cathode cup, were sometimes inaccurately set also degrading the tube's beam-focusing performance. In addition, the tabs were made out of thin sheet metal and acted again like sharp points in the high-voltage field.
In a further effort to eliminate sharp points which could cause internal arcing, a covering cap was traditionally used on the back of the cathode cup to cover the filament lead connections. The cap, having a smooth surface, is less likely to cause internal arcing than the sharp points created where the filament leads are spot welded to power supply wires. However, it was found that the cap itself sometimes had sharp edges and, thus, a separate, smooth guard ring was set in place covering the edges of the cap.
It is, therefore, an object of the present invention to provide a method of manufacturing cathode cups which is substantially simpler than the method used heretofore and which is less prone to cause accidental high-voltage discharge within the X-ray tube.